Nearest celestial neighbours

Talk of the Stars with Professor Don Kurtz

Alpha Centauri is our closest star at only 4.37 light years, or, if you prefer, 40-trillion kilometres away.

From here in Port Alfred look south-southeast over the ocean just after dark in March to see that most iconic of southern constellations: the Southern Cross. Just below the Cross are the two bright “pointer stars”, Alpha and Beta Centauri. The lower, brighter one is Alpha, the brightest star in the constellation Centaurus.

The constellation Centaurus represents the half-man, half-horse “centaur” of Greek mythology. Its origins can be traced back thousands of years to Babylonia, modern-day Iraq. (It takes a vivid imagination to see the shape of the centaur in the constellation.)

But Alpha and Beta Centauri are easy to find and see. Why are they called “Alpha” and “Beta”? They do have traditional Arabic star names, Rigel Kentaurus and Hadar, but we usually name them with Greek letters.

The Greek alphabet begins: alpha, beta, gamma, delta … It is easy to see that the first two letters are the origin of our word “alphabet”.

In 1603 the German Astronomer Johann Beyer produced a new catalogue of the stars and constellations. He decided to name the brightest stars with Greek letters and the constellation name, starting with “alpha” for the brightest, “beta” for the next brightest, and so on.

Hence the brightest star in the constellation Centaurus became “Alpha Centauri”.

But it is more complicated than that!

Alpha Centauri is not just one star. It  is a double star.

The brighter of the two stars is Alpha Centauri A. It has the same surface temperature as our Sun, about 6,000 C, and is about 50% brighter. The fainter of the two stars is Alpha Centauri B, and it is slightly cooler than the Sun at 5,500 C, and only half as bright.

These two stars can be seen individually through a small telescope, but look like only one star to the naked eye.

But Alpha Centauri is even more complicated than that!

It is actually a triple star. The third star is a tiny “red dwarf” known as Proxima Centauri, and it is actually the closest star to the Sun. Proxima is almost too small to be a star and glows with a faint red light, 600 times fainter than our Sun. It takes 550,000 years to orbit the distant AB pair.

Proxima has at least two planets. Excitingly, one is a rocky Earth-sized  planet, Proxima Centauri b, which is at just the right distance from its dim little star to possibly have liquid water, oceans, rivers and lakes. There might even be life there.

When the giant James Webb Space Telescope is launched in October 2021, we will try to detect the atmosphere of this planet to see if there are chemical signs of life. We are not suspecting intelligent life, but even signs of bacteria would be a stunning discovery.

Even more exciting is the “Breakthrough Starshot” project, a wild – but not impossible – idea to launch a swarm of nano-satellites, each the size of a sim card, into orbit about the Earth, then use powerful lasers on the ground to accelerate them to nearly the speed of light. They will be aimed at Proxima and each will carry a tiny camera and radio transmitter.

At nearly the speed of light they will get to Proxima in just over four years, take their pictures quickly as they speed by (there is no stopping them) and beam the pictures back to Earth, where they will arrive after another four years of travel.

Imagine if this can be made to work. In only a couple of decades humans may have their first actual pictures of the surface of a planet orbiting another star. And who knows? Maybe we are not alone.

Have a look at Alpha Centauri tonight, and wonder.


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